Rare earth chalcogenide thermionic emission cathodes

ABSTRACT

This disclosure provides rare earth chalcogenide thermionic emission cathodes. Illustrative examples of such cathodes are GdS and GdSe films.

United States Patent Eastman et al.

[4 1 Jan.18,l972

RARE EARTH CHALCOGENIDE THERMIONIC EMISSION CATHODES inventors: Dean E. Eastman, Route 3, Putnam Valley, N.Y. 10579; Frederick Holtzberg, Cradle Rock Road, Pound Ridge, NY.

10576; Siegfried I. Meihfessel, Hustadtring 24, Bochum 463 W., Germany Filed: Oct. 21, 1970 Appl. No.: 82,659

[56] References Cited UNITED STATES PATENTS 3,139,541 6/1964 Henderson et a1 ..313/346 X 3,273,005 9/1966 Laffeny ..313/346 Primary Examiner-David Schonberg Assistant Examiner-Paul A. Saeher Att0meyHanifin & Jancin and Bernard N. Wiener [57] ABSTRACT This disclosure provides rare earth chalcogenide thermionic emission cathodes. illustrative examples of such cathodes are (ids and GdSe films.

9 Claims, 3 Drawing Figures GdSe [1.5. CI ..3l3/346 Int. Cl. ..H0lj 1/14, H01] 19/06 Field of Search ..313/346 ELECTRON EMISSION AMPS/CM?- PATENTED JAN-1:8IHY2 FIG.1A

FIG. 2

INVENTORS DEAN E. EASTMAN FREDERIC HOLTZBERG SIEGFRIED I. METHFESSEL GdS GdSe l 10- ELECTRON EMISSION AMPS/6M2 BY ATTORNEY RARE EARTH CI-IALCOGENIDE THERMIONIC EMISSION CATIIODES BACKGROUND OF THE INVENTION Thermionic emission is the release of electrons from bodies which are heated. For devices providing thermionic emission, cathodes are of interest which are capable of high current densities. Such thermionic emission cathodes are required for electron guns for microscopes, welding, melting, machining, high-power electron tubes, etc. The following specifications are important for thermionic cathodes: (1) high current densities; (2) stability of emission; (3) long lifetime; (4) low operating temperature; (5) ease of fabrication; (6) ease of activation; (7) chemical stability, i.e., nonreactive with supporting filaments; and (8) stable at high temperatures.

The trivalent rare earth chalcogenides are chemically stable, refractorylike compounds which can be heated to high temperatures. Thermionic emission from LuS, CeS, NdS and PrS is described in the following identified background Russian literature articles:

1. V. I. Marchenko et al., Radiotekhn i Elektron, 8, 1,076

2. V. l. Marchenko et al., Zh. Tekhn, Fiz. 34, 128 (1964). Relatively high work functions and relatively small maximum current densities 10 a./cm. were described therein as having been obtained for each of the four examples.

OBJECTS OF THE INVENTION It is an object of this invention to provide thermionic emission cathodes comprised of the rare earth chalcogenides.

It is another object of this invention to provide GdS and GdSe as thermionic emission cathodes.

SUMMARY OF THE INVENTION It has been discovered for the practice of this invention that the metallic trivalent rare earth chalcogenides RX (R= rare earth =Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, I-Io, Er, Lu; X chalcogen 8, Se, Te) satisfy the requisite conditions for thermionic emission cathodes. They are cathode materials useful for high-current density and long lifetime cathodes.

The trivalent rare earth chalcogenides are utilized for the practice of this invention as high current density thermionic emission cathodes.

Generally, each cathode is used together with a heating element and an electron collecting anode. The anode is placed at a suitable positive electrical potential relative to the cathode. The heating element, which may be the cathode itself, raises the temperature of the cathode to a suitable level resulting in the emission of electrons which are attracted to the collecting anode. Two illustrative classes of devices using such cathodes in accordance with the principles of this invention are electron beam guns and electron microscopes.

In accordance with the principles of this invention, the trivalent rare earth chalcogenides may be used in a variety of cathode configurations. Various cathode configurations suitable for the practice of this invention are presented in U.S. Pat. No. 3,312,856 issued Apr. 4, 1967, to .I. M. Lafferty et al. and in U.S. Pat. No. 3,462,635 issued Aug. 19, 1969 to A. N. Broers.

PRACTICE OF THE INVENTION It has been discovered for the practice of this invention that GdS has a very low work function =2.4 ev. The work function 45 is the minimum energy needed to remove an electron from the Fermi energy, or highest energy state of the metal, into the vacuum outside the metal. It is observed from the Richardson-Dushman equation for thermionic emission,

J(a./cm.)-.47 exp (1 l60/T(K.)) (1) (T= temperature in Kelvin, A Richardson coefficient) that a low work function is an important criterion for efficient electron emission.

It has been discovered for the practice of this invention that GdS and GdSe are desirable thermionic emission cathodes. Because of the chemical similarity of the trivalent rare earths it has been determined for the practice of this invention that various members of the chalcogenides RX, with R trivalent rare earth element and X =chalcogen =S, Se, or Te are good thermionic emission cathodes.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a plane view partially in section of apparatus for practice of this invention and for determining the thennionic emission qualities of the rare earth chalcogenides provided therefor. v

FIG. 1B is a perspective of a portion of FIG. 1A illustrating the heater foil with film of rare earth chalcogenide thereon.

FIG. 2 is a graphical presentation of prior art data on thermionic emission of certain materials and of data concerning GdS and GdSe for the practice of this invention.

PREFERRED EMBODIMENT OF THE INVENTION v The preferred embodiment of this invention will be described as the exemplary design configuration illustrated in FIGS. 1A and 18 used for determining the thermionic emission qualities of the rare earth chalcogenides provided for the practice of this invention. Housing 4, e.g., a glass bell jar, is established on baseplate 6 to form vacuum chamber 5 which is connected to a vacuum pump, not shown, via connection 8. The cathode configuration 9 shown in perspective in FIG. 1B was formed by evaporating a film of rare earth chalcogenide l0, e.g., gadolinium sulfide (GdS) or gadolinium selenide (GdSe) onto front surface 15 of tantalum foil 14. Tantalum foil 14 has connector portions 17 and 18 which are bent-back sections of an original tantalum ribbon of which foil 14 is also a section. Cathode film 10 is heated by the tantalum foil 14 which is heated ohmically using the AC voltage supply 28 via wires 20 and 22. The thermionic electron emission from cathode film 10 is collected by the anode plate 24. The DC voltage 30 is applied between film l0 and anode 24 and the DC current to the anode plate 24 is measured by ammeter32. The thermionic cathode material is characterized by measuring its thermionic emission current I vs. temperature.

For the purpose of measuring the temperature of rare earth chalcogenide film 10, an optical pyrometer 34 is used which monitors the back surface 15 of the tantalum foil 14 through the window 36 in chamber housing 4. The measured thermionic emission characteristic of the preferred embodiment is given by equation (1 presented hereinbefore.

The manner in which the characteristics of exemplary rare earth chalcogenides are measured will now be described with reference to the example GdS. The cathode 10 consisting of the gadolinium sulphide film l0 deposited on the surface 15 of foil 14 is heated with the voltage supply 28 to some temperature determined by observation with the optical pyrometer 34 visually by eye 40, or by a conventional recording instrument, not shown. The true temperature is obtained by correcting the observed temperature by the well-known emissivity factor for tantalum. Alternatively, the optical pyrometer 34 may itself be conventionally calibrated to provide the true temperature for tantalum directly. The resultant current at each temperature is recorded. The heating voltage is then increased until a new temperature is reached and the current is again measured.

A thennionic emission cathode for the practice of this invention has been disclosed herein as including a film 10 ofrare earth chalcogenide structure deposited on a heater foil 14 of tantalum. However, it is within the practice of this invention to have different forms for both the heater means and the rare earth chalcogenide structure. Illustratively, the rare earth chalcogenide structure may be a self-supported segment which is heated via electrical terminals directly connected thereto. It may also have the form of a sintered segment and various other bulk shapes. Further, the heater means may be an electron beam, a laser beam or other electromagnetic radiation beam, e.g., impinging directly on the rare earth chalcogenide structure, or an electrically energized heater other than tantalum which may have various conventional shapes and relationships to the rare earth chalcogenide structure.

A useful figure of merit for high-emission intensity thermionic cathodes is shown in FIG. 2. The evaporation rate is measured in grams/square centimeter/second as a function of electron emission current measured in amperes/square centimeter. This figure of merit is shown in FIG. 2 for several prior art types of high current emission cathodes, i.e., the refractory metals, Mo, Ta and W, as well as LaB (lanthanum hexaboride), a well-known high-emission density thermionic cathode. A background reference for this prior art data is the article by J. M. Lafferty, Journal of Applied Physics, 22, p. 299-309 (1951). Measurements are shown on FIG. 2 for the rare earth chalcogenides GdS and GdSe in accordance with the practice of this invention. It is noted that at the same emission current density, i.e., approximately 2 a./cm. GdS has a superior figure of merit to all of the refractory metals and is inferior only to the lanthanum hexaboride LaB The data in FIG. 1 for GdS and GdSe is shown by a vertical line with two horizontal error bars. These error bars depict the range of evaporation rate which has been determined for GdS and GdSe as examples of this invention, i.e.,' the measurements are uncertain within the limits of the vertical distance on the graph between the error bars.

EXAMPLES OF THE INVENTION A thermionic emission cathode 10 (FIGS. 1A and 1B) was prepared by evaporating a film of GdS, about 1,000 A. thick, on a heated 3-mil thick tantalum ribbon 14 of 2X5 mm. active area. The cathodes were stored for several days in a desiccator and mounted in another vacuum system (FIG. 1A) for measurement-of the thermionic emission characteristics. A diffusion pump was used to establish a vacuum in chamber 5. Pressures were approximately 6X 1 torr during measurements.

The GdS cathodes 10 were easily activated by ohmic heating of the Ta ribbon 14 to l,400 C. for less than 1 minute and showed stable and reproducible emission. The temperature was measured on the rear surface of the Ta ribbon with an optical pyrometer 34, and was corrected for radiative emissivity. The resultant thermionic emission cathode configuration 9 (FIG. 13) gave a high electron emission intensity of i=0. 1 8 a. (i.e., 1.8 a./cm.) at an operating temperature of l,620 C. The temperature dependence is given approximately by equation 1) presented hereinbefore as:

l(a.)A'T(K.) exp(l,l60rp/T(K.)) (l) with A=l.45 a./cm./"l(. and d =2.4 ev. with an active GdS area of 0.] cm.. The temperature is measured in degrees Kelvin. The high thermionic emission intensity of metallic GdS is due in part to a low work function =2.4 ev. combined with a high melting point. lllustratively, other conventional substrate metals such as W, lr, and Mo may be used in place of Ta for the practice of this invention.

For comparison with other cathodes, GdS has an electron emission (a./cm.) per evaporation rate (g./cm. /sec.) figure of merit which is superior to the standard refractory metals Mo, W, and Ta. This is shown in FIG; 2, where measurements for GdS and GdSe are compared with other known high current density cathodes. The hexaborides [.28. and YB, (not shown) have higher figures of merit, but have serious high temperature chemical reactions with refractory metal support filaments.

Another significant advantage of the rare earth chalcogenides, e.g., GdS and GdSe, over other high-emission materials such as the LaB, compounds is the ease of fabrication. The rare earth chalcogenides are readily prepared by evaporation, while the LaB, compounds cannot be prepared readily, if at all, by evaporation. A background literature reference concerning thermionic emission from Lat is the noted article by J. M. Lafferty, Journal of Applied Physics, 22, p. 299-309 (1951). Further, arbitrarily shaped cathodes of the rare earth chalcogenides can be formed by machining.

An exemplary thermionic emission cathode of GdSe has also been prepared in accordance with the principle of this invention. Very similar emission characteristics were obtained. Emission from GdSe is described by nearly the same A and d: constants in the Richardson-Bushman equation (I) as for GdS.

What is claimed is:

l. A thermionic emission cathode comprising a structure of rare earth chalcogenide RX selected from the group consisting of R =trivalent rare earth =Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, H0, Er or'Lu, and X =chalcogen=S, Se or Te.

2. A thermionic emission cathode as set forth in claim 1 wherein said rare earth chalcogenide is GdS.

3. A thermionic emission cathode as set forth in claim 1 wherein said rare earth chalcogenide is GdSe.

4. A thermionic emission cathode as set forth in claim 1 wherein said rare earth chalcogenide structure is selected from the group consisting of film, sintered powder segment, and bulk form.

5. A device for providing thermionic electrons comprising:

a structure of a rare earth chalcogenide RX selected from the group consisting of R trivalent rare earth =Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er or Lu, and X chalcogen=S, Se or Te,

means for causing said structure to emit thermionic electrons as a cathode; and

anode means for receiving said thermionic electrons.

6. A device as set forth in claim 5 wherein said rare earth chalcogenide is GdS.

7. A device as set forth in claim 5 wherein said rare earth chalcogenide is GdSe.

8. A device as set forth in claim 5 wherein said rare earth chalcogenide structure is selected from the group consisting of film, sintered powder segment, and bulk form. g

9. A device as set forth in claim 5 wherein said means for causing said structure to emit thermionic electrons is selected from the group consisting of: i g

a. an electrically energized heater means in direct contact with said structure;

b. an electrically energized heater means in proximity to' said structure;

c. electrical energizing means for heating said structure ohmically;

d. electron beam means for heating said structure with an electron beam; and

e. electromagnetic radiation beam means for heating said structure with electromagnetic radiation beam. 

2. A thermionic emission cathode as set forth in claim 1 wherein said rare earth chalcogenide is GdS.
 3. A thermionic emission cathode as set forth in claim 1 wherein said rare earth chalcogenide is GdSe.
 4. A thermionic emission cathode as set forth in claim 1 wherein said rare earth chalcogenide structure is selected from the group consisting of film, sintered powder segment, and bulk form.
 5. A device for providing thermionic electrons comprising: a structure of a rare earth chalcogenide RX selected from the group consisting of R trivalent rare earth Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er or Lu, and X chalcogen S, Se or Te, means for causing said structure to emit thermionic electrons as a cathode; and anode means for receiving said thermionic electrons.
 6. A device as set forth in claim 5 wherein said rare earth chalcogenide is GdS.
 7. A device as set forth in claim 5 wherein said rare earth chalcogenide is GdSe.
 8. A device as set forth in claim 5 wherein said rare earth chalcogenide structure is selected from the group consisting of film, sintered powder segment, and bulk form.
 9. A device as set forth in claim 5 wherein said means for causing said structure to emit thermionic electrons is selected from the group consisting of: a. an electrically energized heater means in direct contact with said structure; b. an electrically energized heater means in proximity to said structure; c. electrical energizing means for heating said structure ohmically; d. electron beam means for heating said structure with an electron beam; and e. electromagnetic radiation beam means for heating said structure with electromagnetic radiation beam. 